Semiconductor lasers are widely used in applications such as optical communications, optical disk players, etc. A typical semiconductor laser contains an active region disposed between two parallel mirrors which form therebetween an optical cavity. When an electrical current is applied, the active region generates optical radiation which is reflected back and forth between the mirrors before it is emitted. When the applied current is greater than a threshold current of the laser, the optical radiation in the optical cavity coherently oscillates to form a standing wave. As a result, the laser emits coherent optical radiation.
The optical radiation emitted from the laser may have different spatial distributions. If a laser emits optical radiation that contains only the fundamental transverse mode, the radiation is a narrow laser beam with a Gaussian-distributed cross-section, most intense in the center and less intense at the edges. In contrast, laser radiation having a higher transverse mode displays bright and dark spots across the cross-section of the radiation. The transverse modes are conventionally designated as TEM.sub.00, TEM.sub.01, TEM.sub.10, TEM.sub.11, etc. where TEM.sub.00 is the fundamental transverse mode and the others are higher transverse modes.
Radiation with higher transverse modes is normally undesirable because it is difficult to couple such radiation into optical fibers and to focus it for free-space beam forming. In addition, higher transverse mode radiation travels at somewhat slower speed in an optical fiber than the fundamental transverse mode radiation, thereby creating mode dispersion, i.e., broadening of an optical pulse as it travels in an optical fiber.
In a vertical cavity surface emitting laser (VCSEL), the mirrors that form the optical cavity are parallel to a substrate on which the laser is formed. Thus, the optical cavity of a VCSEL is perpendicular to the substrate; and optical radiation is emitted from the VCSEL in a direction normal to the substrate.
VCSELs have many advantages over conventional edge emitting lasers. For example, VCSELs can be made extremely small; VCSELs can easily be made into arrays that contain a large number of VCSELs; and VCSELs can be readily integrated monolithically with other semiconductor devices.
However, currently available VCSELs have several problems. One problem relates to higher transverse mode lasing. A typical circular shaped VCSEL with a diameter greater than 10 .mu.m emits TEM.sub.00 mode radiation only at low current. At high current, the VCSEL emits higher transverse mode radiation.
Second, unlike edge emitting lasers, available VCSELs emit radiation having uncontrolled directions of polarization. In many applications (e.g. magneto-optical disks), lasers having controlled directions of polarization are highly desirable.
Third, currently available VCSEL arrays cannot be made to emit laser beams, each of which has a predetermined orientation of polarization. Adjacent VCSELs in a VCSEL array have a tendency to couple with each other. In some instances, this results in unwanted beam cross sections. It could be prevented if it were possible to control the direction of polarization of adjacent VCSELs since adjacent VCSELs that have perpendicular polarizations usually couple weakly. In other instances, it may be desirable to control the directions of polarization of the VCSELs so that they all have the same direction of polarizations, i.e., have parallel polarizations.
It is therefore an object of the present invention to provide a semiconductor laser that emits optical radiation having a controlled direction of polarization.
It is another object of the present invention to provide a semiconductor laser that emits optical radiation in substantially the fundamental TEM.sub.00 mode.
It is yet another object of the present invention to provide a VCSEL array in which the directions of polarizations of adjacent VCSELs can be predetermined.